PESTICIDE RESIDUE • 105
Parts Per Million. Pesticide residues are normally measured in parts per million (ppm). One part per million equals one part of a chemical per million parts of the commodity. In general, pesticide residues are measur- able at a minimum detectable level of 0.01 ppm.
This is also what happens if it is found to have a residue for which there is no tolerance established. The grower is then subject to prosecution, and so are the packer and shipper. The quaran- tined lot cannot be moved or disposed of without approval from DPR.
Tolerances for Combinations of Pesticide Residues
Agricultural commodities can be quarantined or prohibited from harvest if they are found to carry an unaccept- able residue level from a combination of related or unrelated pesticides. See Sidebar 11 for examples of how combinations of related and unrelated residues can add up to legal or illegal tolerance levels.
When an agricultural commodity is found to carry pesticide residues in excess of the legal tolerance, the commodity is quarantined and re- moved from sale and distribution.
If you treat a crop, commodity, or site with a pesticide that is not registered for use on that crop, commodity, or site, it is considered a public nuisance. It may be seized by the Director of DPR to prevent its harvest or sale and to prevent planting of the site.
Many growers and chemical firms use the services of commercial labora- tories that provide pesticide residue analysis. All laboratories that analyze produce for pesticide residues must be accredited by the California Depart- ment of Health Services. You can obtain the names and locations of these laboratories by contacting your local county agricultural commissioner.
Tolerances for Combinations of Related and Unrelated Pesticide Residues
Related Pesticide Residue Combination Consider a lettuce crop treated with pesticide X and pesticide Y, both organophosphates. The established tolerance for pesticide X on lettuce is 2 ppm, and the established tolerance for pesticide Y is 7 ppm. One lot of packed lettuce was found to carry 1 ppm of pesticide X and 4 ppm of pesticide Y. To determine the percent of allowable tolerance, related chemi- cal residues are added on a percentage basis:
Residue in Percent of Tolerance
One ppm is 50 percent of 2 ppm, and 4 ppm is 57.1 percent of 7 ppm. When 50 percent and 57.1 percent are added together, the result is 107.1 percent. This lot is in violation, because the combined residues exceed 100 percent of the allowed tolerance for organophosphates.
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